The present invention relates to a plate pack for a plate heat exchanger, comprising a number of heat transfer plates, each of which has a heat transfer portion and a number of through ports, said plates interacting in such manner, that a first flow duct is formed between them in a plurality of first plate interspaces and a second flow duct is formed in a plurality of second plate interspaces and that the ports form at least one inlet duct and at least one outlet duct for each of the flow ducts, that the inlet duct of at least the first flow duct comprises at least one primary duct, which is arranged to receive a fluid flow for the first flow duct, and at least one secondary duct, which communicates with the primary duct and the first flow duct and which is arranged to receive the fluid flow from the primary duct and to convey the fluid flow to the first flow duct.
The invention further relates to a flow distribution device for use in an inlet duct of a plate heat exchanger, and a plate heat exchanger.
A plate heat exchanger may comprise a xe2x80x9cframe platexe2x80x9d, a xe2x80x9cpressure platexe2x80x9d and a number of intermediate heat transfer plates clamped together in a xe2x80x9cplate packxe2x80x9d. The heat transfer plates are arranged and designed so that flow paths for at least two heat transfer media are formed between them. Each heat transfer plate is provided with a number of through ports, which together form at least two inlet ducts and two outlet ducts extending through the plate pack. One of the inlet ducts and one of the outlet ducts communicate with each other via some of the flow paths, which form a flow duct for one heat transfer medium, and the other inlet and outlet ducts communicate with each other via the other flow paths, which form a flow duct for another heat transfer medium.
The plate heat exchanger works by two different heat exchanging media being supplied, each via a separate inlet duct, to two separate flow ducts, where the warmer medium transfers part of its heat content to the other medium by means of heat transfer plates. The two media can be different liquids, gases, vapours or combinations thereof, so-called two-phase media.
The plate heat exchanger concept will be described in more detail in connection with a plate heat exchanger intended for so-called two-phase application and described in the Alfa Laval AB brochure The plate evaporator from 1991 (IB 67068E) (see FIG. 1).
The medium that is to be completely or partially vaporised, for example juice that is to be concentrated, is supplied to the heat exchanger through an inlet formed by two openings in the frame plate. These two openings lead directly to a common first inlet duct, which extends through the pack of heat transfer plates. Vapour is supplied to the flow ducts formed between the heat transfer plates and intended for this purpose through a second inlet duct. This second inlet duct is formed by ports located in an upper corner portion of the plates and, since the vapour takes up a relatively large volume, it has a relatively large cross-sectional area.
When the plate heat exchanger is in operation the vapour flows downwards in its interspaces and is completely or partially condensed. The condensate is discharged through two outlet ducts, which are defined by ports in the two lower corners of the plates and which lead out from the plate heat exchanger via two connecting ports in the frame plate. The second medium is conveyed upwards in its interspaces and is completely or partially vaporised before being finally discharged via an outlet duct, which is formed by ports located in the other upper corner of the plates and which leads out via a connecting port in the frame plate.
A problem associated with this technique is that in long plate heat exchangers, i.e. plate heat exchangers with a large number of heat transfer plates in the plate pack, the amount of flow of the two media in the plate interspaces tends to vary along the length of the plate heat exchanger. Therefore, the maximum capacity of the plate heat exchanger cannot be exploited. Even if one or several plate interspaces are utilised at maximum capacity, there is a fairly large number of plate interspaces whose utilisation level is considerably below the maximum capacity. This problem is accentuated in two-phase applications, since the vapour phase of a medium has different characteristics than the liquid phase. This means that the vapour phase and the liquid phase will behave differently in the heat exchanger and thus present a different distribution in the plate interspaces concerned.
Another problem associated with most plate heat exchangers is that it is difficult, in many cases, to obtain an even distribution of the fluid flow across the whole width of each plate, i.e. across the entire heat transfer portion. One way to try to improve the distribution is to give the plate ports intended to form the inlet duct an elongate shape, as shown in FIG. 1. To facilitate connection of the heat exchanger to other devices it is possible to use, for instance, two connecting ports in the frame plate, which connect directly to the inlet duct having an elongated cross-section. In general, it is undesirable to have such abrupt dimensional variations in a duct. Because of the dead flow space formed immediately behind the connecting ports of the frame plate, the first interspaces do not get the desired distribution of liquid. Instead any gases present have a tendency to flow in these plate interspaces.
The above-related problems arise even if the plate heat exchanger is not being used for two-phase applications, but they are particularly pronounced in two-phase applications.
WO97/15797 discloses a plate heat exchanger, which is intended for evaporation of a liquid, for example a refrigerant. This plate heat exchanger has an inlet duct and a distribution duct, which extend through the plate heat exchanger and communicate with each other along the whole length of the plate heat exchanger. The purpose of the distribution duct is, inter alia, to create substantially equal flows in the different plate interspaces by serving as an expansion or equalization chamber between the inlet duct and the plate interspaces. The proposed design does not, however, provide a completely satisfying solution for all operational situations in which conventional industrial plate heat exchangers are used.
GB-A-2 052 723 and GB-A-2 054 124 disclose two variants of a plate heat exchanger having a front and a rear section of plate interspaces. To allow the flow to the plate heat exchanger to reach the rear section, the plate heat exchanger is provided with a by-pass duct consisting of a pipe, which is concentrically arranged in the inlet duct. The purpose of the concentric pipe is to convey part of the flow to the rear section. The plate interspaces of the first section communicate directly with the front portion of the inlet duct. The plate interspaces of the second section communicate directly with the rear portion of the inlet duct.
Consequently, there are no prior art constructions, which give a satisfactory flow distribution both along the length of the plate heat exchanger and across the width of the plates. Above all, there is no prior art construction that solves these problems in two-phase applications.
The object of the invention is to provide a solution, which allows a satisfactory flow distribution along the length of the plate heat exchanger and across the width of the plates, and by means of which it is also possible to avoid the above distribution problems in two-phase applications.
The present object is achieved by means of a plate pack of the type described by way of introduction, characterised in that the primary duct and the secondary duct communicate with each other through at least one flow passage portion spanning a plurality of plate interspaces, that the extension of the flow passage portion along the primary duct is substantially smaller than the extension of the primary duct, and that there is substantially no flow passage between the primary and secondary ducts outside said flow passage portion.
By providing the plate pack with one primary and one secondary duct, and supplementing this configuration with the flow passage described above, a plate pack in which the fluid flow can be advantageously distributed both along the length of the plate pack and across the width of the plates is obtained. The fluid flow which has flowed from the primary duct through the flow passage will whirl around in the secondary duct, largely because of the limited extension of the flow passage, and will thus be evenly distributed along the length of the plate pack. By controlling in which plate interspaces the fluid will flow between the primary and secondary ducts, the flow in the secondary duct can be controlled and thus the flow distribution across the length of the plate pack. In addition, the limited extension of the flow passage portion along the length of the primary duct means that different fluid phases will not prefer different ways between the primary and secondary ducts, but substantially the same phase distribution as that of the two-phase fluid in the primary duct will flow to the secondary duct and, through this, be distributed between the different plate interspaces. Owing to the use of one primary duct and one secondary duct, the secondary duct may further be designed to spread the fluid flow across the entire width of each plate, whereas the primary duct may be designed to allow conventional, round pipes to be connected to the plate pack. By providing the inlet duct with one primary duct and one secondary duct, the interface between duct and heat transfer surface and the interface between duct and external connections can be designed relatively independently from each other. This means that abrupt dimensional variations in the flow paths can be avoided, and thus also any undesirable turbulence.
Preferred embodiments of the invention are apparent from the dependent claims.
According to a preferred embodiment, the primary duct communicates with the secondary duct through at least two flow passage portions located at a distance from each other along the primary duct. This means that a fluid flow can be distributed across long plate packs while maintaining the positive distribution properties described above. This embodiment also provides a large amount of flexibility as regards different forms of sectioning of the plate pack.
According to a further preferred embodiment, a flow distribution device is arranged in the primary duct for deflecting part of the fluid flow in the primary duct via said flow passage portion. By arranging a flow distribution device in the primary duct, the amount of fluid flow deflected from the primary duct in different places along the primary duct can be regulated in a simple and reliable way. The deflecting property of the flow distribution device also stimulates the equalizing fluid flow in the secondary duct.
The primary duct advantageously extends through the whole plate pack, since this is a simple way of supplying the whole plate pack with fluid.
According to a preferred embodiment, the secondary duct extends through the whole plate pack. Owing to this design only one secondary duct is needed for the whole plate pack.
According to a preferred embodiment, the secondary duct is divided into a number of separate sections, each extending only through part of the plate pack. This design is particularly suitable in plate packs consisting of a large number of plates, and it makes it possible to obtain an equalization of the fluid flow for a determined number of plate interspaces in each secondary duct. By distributing the equalizing function among a number of separate secondary ducts, a slightly lower degree of equalization for each of the secondary duct sections can be tolerated, while still obtaining a satisfactory distribution along the whole length of the plate pack, than what would have been possible with a single long secondary duct with the same degree of equalization. This division means that the plate pack can be used in more varying applications without major performance losses.
The flow distribution device suitably delimits a section of the cross-sectional area of the primary duct, which section is reduced along the primary duct in the flow direction of the fluid flow. The flow deflected from the primary duct is thereby supplied to the secondary duct in a way that is consistent with fluid technology.
According to a preferred embodiment, the flow distribution device comprises a tubular body surrounding an inclined ramp. The tubular shape of the body allows it to be easily arranged and fixed in the inlet duct of the plate pack. The inclined ramp provides a good deflecting action, since it allows the fluid to flow along the ramp in such manner that its flow direction is gradually redirected.
The front portion of the inclined ramp is advantageously located at a distance from the duct wall of the primary duct. This ensures that the ramp extends into the fluid flow of the duct and deflects part of the flow.
The rear portion of the inclined ramp suitably connects to the duct wall of the primary duct adjacent to the flow passage between the primary duct and the secondary duct. This results in the deflected fluid flow being conveyed directly to the secondary duct.
An appropriate way of reliably deflecting a correct share of the fluid flow is to provide the inclined ramp of the flow distribution device with a deflecting edge, which is oriented in a direction opposite to the fluid flow.
According to a preferred embodiment, the deflecting edge extends essentially vertically. This orientation of the deflecting edge is advantageous in that also two-phase flows, such as annular or stratified flows, are divided into approximately equal shares of each of the different phases. This is important since an uneven distribution of vapour and liquid, respectively, both reduces the capacity of the plate heat exchanger and increases the risk of the heat exchanger xe2x80x9crunning dryxe2x80x9d, i.e. that the fluid flow between one or several plates is not sufficient, which may cause solid particles in the fluid flow to get burnt and stick to the plates.
The inclined ramp suitably comprises an essentially flat, semi-elliptical sheet. This is a simple way of ensuring the deflecting action of the flow distribution device.
The extension of the inclined ramp along the primary duct is advantageously larger than its largest extension across the primary duct. As a result, the deflection obtained does not cause any extensive turbulence.
According to a preferred embodiment, the flow distribution device comprises a number of outwardly extending connecting means arranged to be fixed between the plates in their abutment against each other round the primary duct. By fixing the flow distribution device in this way no supplementary means for fixing the flow distribution device in the duct are needed. The forces of the tie bars acting to compress the plate pack are thereby also used to fix the flow distribution device.
According to a preferred embodiment of the body, it comprises an open, tubular cage structure, which surrounds and supports the inclined ramp. The body thus surrounding the ramp facilitates a correct positioning of the ramp in the duct.
According to a preferred embodiment, the body comprises a pipe, which surrounds the inclined ramp and which is provided with an opening in its circumferential surface, the inclined ramp being connected to said opening. This body design is very robust and does not affect the fluid flow in the duct very much. It also ensures that correct shares of the fluid are conveyed to the secondary duct. The tubular shape ensures that unwanted leaks between primary and secondary ducts are avoided.
The external shape of the flow distribution device suitably corresponds to the internal shape of the primary duct. This means that the flow distributor interferes only to a very small extent with the fluid flow, and because more or less coincident surfaces can be used, that it is easier to obtain a correct positioning.
According to a preferred embodiment, the flow passage between the primary duct and the secondary duct has an extension length along the primary and secondary ducts that is smaller than the extension length of each of the ducts along each other. This construction enhances the tendency of the fluid flow to present an equalizing, circulating flow in the secondary duct, resulting in an excellent distribution across the different plate interspaces communicating with the secondary duct.
According to a preferred embodiment, there is only one flow passage between the primary and the secondary duct. This enhances the tendency of the fluid flow to present an equalizing, circulating flow in the secondary duct.
By using a plate pack of the kind described above in a plate heat exchanger, a plate heat exchanger in which the fluid flow is evenly distributed across the different plate interspaces is obtained. The even distribution will also be obtained in two-phase applications, i.e. when the fluid has both liquid and gas phases. The primary duct, with its flow distribution device conveys the fluid flow to the secondary duct, where the fluid flow is equalized.
According to a preferred embodiment, the plate heat exchanger comprises at least two plate packs, wherein the primary duct of the first plate pack is connected to and substantially coincides with the primary duct of the second plate pack, and the secondary duct of the first plate pack is separated from the secondary duct of the second plate pack. This construction gives a very favourable distribution of the fluid flow along the length of the plate heat exchanger even if a somewhat less satisfactory distribution would be obtained locally in a plate pack.